1. Field of the Invention
The present invention relates to electronic integrated circuits. More specifically, the present invention relates to electronic integrated circuits including a temperature sensor.
2. Description of the Related Art
Microprocessor architectures are continually evolving to improve and extend the capabilities of personal computers. Execution speed, power consumption, and circuit size are aspects of microprocessors and microprocessor performance that are constantly addressed by processor architects and designers in the ongoing quest for an improved product. As microprocessor circuits grow in size, increase in density, and accelerate in execution speed, exceeding 200-500 MHz and continuing to increase, the maintenance of a suitable operating temperature is an important design consideration.
Maintaining a suitable operating temperature is highly important design consideration in microprocessor-based computer systems. Designers typically use mechanical devices, such as fans or heat sinks, to maintain a cool operating temperature. Some systems, such as many portable computer systems, do not use a fan or do not have sufficient area to house a large heat sink so that the microprocessor circuit often operates near to the upper range of operating temperature limits. When a microprocessor operates at a temperature above the operating limit, data errors or reliability problems may occur due to stress on operating speed paths.
To avoid such errors, a computer system may include a temperature sensor to monitor the internal environment of the computer system. If the temperature reaches a level above prescribed limits, automatic temperature management utilities may be invoked to disable clock signals, actuate a cooling fan, or other similar operations, to reduce the operating temperature to a suitable range.
A first type of conventional temperature sensors, including digital thermometers, typically measure temperature by exploiting the thermal-voltage characteristics of a diode. The voltage drop across a diode falls as a function of temperature as the diode is forward-biased by a constant current. The voltage-temperature relationship reflects a temperature coefficient that may be employed to measure temperature in a temperature sensor. The temperature coefficient of a diode is generally a constant value illustrating a relationship that voltage decreases as temperature increases. A typical temperature coefficient is about -2 mV/.degree. C. The first type of conventional temperature sensors normally generate an output signal that increases with increasing temperature. The voltage drop across a diode does not supply a suitable output signal since it decreases with increasing temperature.
In one embodiment of a diode-based temperature sensor, a diode is constructed as a source-drain of an output driver. The diode has a temperature dependence so that a voltage placed on one terminal of the diode results in a measurable current flow that is a function of temperature. Unfortunately, the current is an exponential function of voltage, so that any noise, such as digital noise occurring during operation of a microprocessor, is amplified in an exponential manner so that noise signals completely overpower voltage signals arising from temperature. Generally, an accurate temperature measurement is performed only by terminating all operations of a circuit, allowing signals to settle, then making a temperature measurement reading. The decay of the current is tracked and a curve is fit to determine the temperature at the time operations are terminated. The diode measurement technique is complicated and accurate measurements are difficult to achieve. Real-time measurements are virtually impossible to achieve.
A second type of conventional temperature sensor generates a voltage drop of two diodes with different current densities, either by applying the same current to different sized diodes or by applying different currents to identically sized diodes. The diode with a higher current density has a relatively smaller absolute value temperature coefficient than the other diode. However, the two diodes have the same voltage drop at absolute zero. As temperature increases, the relative difference between the voltage drops of the two diodes increases in a linear manner. The conventional temperature sensor includes a differential amplifier that receives the voltage drops of the diodes and generates an output signal representing the difference between the voltage drops. The output signal linearly increases as temperature increases.
CMOS technology is typically used to fabricate microprocessors due to considerations of low DC power dissipation, high noise margin, wide temperature and voltage ranges, overall circuit simplification, layout ease, and high packing density. In contrast, bipolar technology is generally used to construct temperature sensors, using diode-connected bipolar transistors to provide the temperature coefficient as base-emitter voltage (V.sub.BE) as a function of temperature. Therefore, temperature sensors and microprocessors are usually implemented in separate technologies with the temperature sensor positioned one or two inches from the microprocessor due to packaging constraints. The distance between the microprocessor and the temperature sensor greatly reduces the accuracy of the microprocessor temperature operating condition. The temperature sensed by the temperature sensor is skewed by the ambient temperature and the temperature of circuits nearer to the sensor than the microprocessor.
BiCMOS technology apparently would allow integration of CMOS and bipolar technologies feasible, but a tradeoff between process complexity and device quality generally renders BiCMOS unacceptable for microprocessor usage.
A third temperature sensing technique employs a ring oscillator that is very sensitive to temperature differences. The output signal from the ring oscillator is buffered and the frequency of the oscillator output signal is measured. The circuit temperature varies based on the measured frequency. Unfortunately, the functionality of the ring oscillator is highly process dependent and varies with several variables including polysilicon resistance and channel length. To accurately calibrate a ring oscillator temperature sensor, the individual devices are to be characterized, the characterization parameters stored, and calibration operations carried out, greatly complicating the measurement of temperature in a production environment.
What is needed is an improved temperature sensor that more accurately senses a microprocessor temperature than conventional approaches.